The present invention relates to a gas sensor and a gas sensor system using the same.
Resistance-type sensors are known for measuring the concentration of a combustible gas component such as hydrocarbon (hereinafter may be referred to as HC) or CO contained in exhaust gas from an automobile or the like. For example, an oxide semiconductor (n type) such as SnO2 or the like is used as a sensing element for measuring the concentration of a combustible gas component such as HC or CO. Specifically, oxygen in the atmosphere adsorbs on the sensing element through an effect induced by negative charges. When the atmosphere contains a combustible gas component such as HC or CO, the combustible gas component undergoes a combustion reaction with the adsorbing oxygen, thereby causing oxygen to be desorbed from the sensing element. Since a change in an electric resistance of the sensing element associated with the oxygen desorption depends on the combustible gas component concentration of the atmosphere, the combustible gas component concentration of the atmosphere can be obtained through measurement of the change of the electric resistance. However, such a resistance-type sensor has a drawback that an output from the sensing element formed of an oxide semiconductor is likely to vary depending on the concentration of oxygen or water vapor contained in an exhaust gas. Accordingly, even when the combustible gas component concentration remains unchanged, a detection output value varies depending on, for example, the oxygen concentration of the exhaust gas.
In order to solve the above problem, an apparatus for measuring a combustible gas component concentration having the following structure is disclosed in Japanese Patent Application Laid-Open No. 8-247995. In the apparatus, a sensing element has two processing zones. An exhaust gas is introduced into a first processing zone via first diffusion-controlling means. Oxygen is pumped out from the first processing zone by a first oxygen concentration adjustment pump element so as to reduce the oxygen concentration of the first processing zone to a low value at which a combustible gas component is not substantially burned. Next, the gas having the thus-reduced oxygen concentration is introduced into a second processing zone via second diffusion-controlling means. Oxygen is pumped into the second processing zone by a second oxygen concentration adjustment pump element so as to burn the combustible gas component. The combustible gas component concentration is determined based on a value of current flowing through or voltage built up across the second oxygen concentration adjustment pump element.
The apparatus disclosed in the above-described patent publication employs a structure such that oxygen is pumped into the second processing zone by the second oxygen pump element in order to burn the combustible gas component on the electrode in the second processing zone. The oxygen concentration in the second processing zone is detected by the oxygen concentration cell element, and the second oxygen pump element is operated such that the detected electromotive force becomes constant. The concentration of the combustible gas component is detected on the basis of the pumping current of the second oxygen pump element. However, this structure has the following drawback. When the oxygen concentration changes as a result of combustion of the combustible gas component, this change is detected by the oxygen concentration cell element and is fed back to a control section for controlling the pumping current, so that the response in detecting the combustible gas component decreases accordingly.
Further, in the above disclosed apparatus, the oxygen concentration of the exhaust gas introduced into the first processing zone is reduced by the first oxygen concentration adjustment pump element to xe2x80x9ca low value at which a combustible gas component is not substantially burned.xe2x80x9d According to the publication, the low value is not higher than 10xe2x88x9214 atm, preferably not higher than 10xe2x88x9216 atm, and is normally about 10xe2x88x9220 atm. However, when the oxygen concentration of the first processing zone is set at such a low value, there arises the following problem related to accuracy in measuring the combustible gas component concentration.
Specifically, an exhaust gas generally contains a fair amount of water vapor in addition to combustible gas components such as hydrocarbon, carbon monoxide, and hydrogen. Generally, the amount of water vapor varies according to operating conditions of an internal combustion engine. According to studies conducted by the inventors of the present invention, when the oxygen concentration of such an exhaust gas is reduced to the above-mentioned value, a portion of water vapor is decomposed into hydrogen and oxygen. The thus-generated oxygen is pumped out from the first processing zone by the first oxygen concentration adjustment pump element, whereas the thus-generated hydrogen is not pumped out, but introduced into the second processing zone, where the hydrogen induces combustion. If such a state occurs during measurement of a gas to be examined which mainly contains hydrocarbon as a combustible gas component, the accuracy in measuring hydrocarbon concentration is greatly affected by combustion of hydrogen generated through decomposition of water vapor. Notably, measurement examples disclosed in the above publication are all conducted under the condition that the water vapor concentration of the gas to be examined is constant, and do not refer to the influence of a variation in water vapor concentration on measurement of a combustible gas component concentration.
As disclosed in the above publication, a proton pump may be additionally used in order to pump out the thus-generation hydrogen from the first processing zone, so that only HC is selectively burned to thereby improve measurement accuracy. However, this method merely employs the proton pump as a means of last resort for coping with hydrogen generation associated with decomposition of water vapor. Addition of the proton pump makes a sensor structure and a sensor control mechanism complex, causing an increase in apparatus cost. Further, residual hydrogen which the proton pump has failed to pump out may induce a measurement error.
Also, the following problem is involved. With the recent tendency to tighten exhaust gas regulations for air pollution control, internal combustion engines such as gasoline engines, diesel engines, and like engines tend to shift to the lean-burn type in order to suppress generation of HC associated with incomplete combustion. An exhaust gas produced under lean-burn conditions has an oxygen concentration higher than that produced under stoichiometric or rich conditions. When the above-mentioned conventional apparatus is applied to such an exhaust gas, an oxygen concentration adjustment pump element carries a significant burden in order to reduce the oxygen concentration to the above-mentioned low value. As a result, the service life of the oxygen concentration adjustment pump element is shortened. Further, since the operating power of the oxygen concentration adjustment pump element must be increased, a peripheral control circuit must be of high output, causing an increase in apparatus cost.
A first object of the present invention is to provide a gas sensor which can detect the concentration of a combustible gas component in a measurement gas, such as exhaust gas, with high accuracy even when the oxygen concentration of the measurement gas or the element temperature changes and which has an excellent response in detecting the combustible gas component, as well as to provide a gas sensor system using the gas sensor. An second object of the present invention is to provide a gas sensor in which accuracy in measuring a combustible gas component concentration is less susceptible to decomposition of water vapor and which is suitably applicable to lean-burn conditions, as well as to provide a gas sensor system using the gas sensor.
The gas sensor of the present invention has the following constituent features.
1. First processing space: Isolated from surroundings. A measurement gas is introduced into the first processing space via a first gas passage.
2. Second processing space: Isolated from surroundings. A gas contained in the first processing space is introduced into the second processing space via a second gas passage.
3. Oxygen concentration detection element: Adapted to measure the oxygen concentration of a gas contained in the first processing space.
4. Oxygen concentration adjustment pump element: Formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. The oxygen concentration adjustment pump element pumps out oxygen from the first processing space or pumps oxygen into the first processing space so as to adjust to a predetermined level the oxygen concentration of the measurement gas introduced into the first processing space, which is detected by the oxygen concentration detection element.
5. Oxidation catalyst section: Adapted to accelerate combustion of a combustible gas component contained in the gas which has been introduced into the second processing space from the first processing space via the second gas passage.
6. Measurement-purpose-oxygen supply pump element: Formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. One of the electrodes is disposed to be exposed to the second processing space. A voltage is applied between the electrodes in order to pump into the second processing space a constant amount of oxygen used for measurement of the combustible gas component contained in the measurement gas.
7. Combustible gas component concentration detection element: Formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. One of the electrodes is disposed to be exposed to the second processing space. A constant voltage is applied between the electrodes in a direction for pumping out oxygen from the second processing space (i.e., a direction such that the electrode facing the second processing space become negative), so that a current output from the combustible gas component concentration detection element varies according to the amount of oxygen consumed by combustion of the combustible gas component contained in the gas introduced from the first processing space into the second processing space to thereby provide information regarding a detected concentration of the combustible gas component of the measurement gas.
A gas sensor system of the present invention has the following constituent features.
(A) Gas sensor: Configured to have the following constituent features.
1. First processing space: Isolated from surroundings. A measurement gas is introduced into the first processing space via a first gas passage.
2. Second processing space: Isolated from surroundings. A gas contained in the first processing space is introduced into the second processing space via a second gas passage.
3. Oxygen concentration detection element: Adapted to measure the oxygen concentration of a gas contained in the first processing space.
4. Oxygen concentration adjustment pump element: Formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. The oxygen concentration adjustment pump element pumps out oxygen from the first processing space or pumps oxygen into the first processing space.
5. Oxidation catalyst section: Adapted to accelerate combustion of a combustible gas component contained in the gas which has been introduced into the second processing space from the first processing space via the second gas passage.
6. Measurement-purpose-oxygen supply pump element: Formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. One of the electrodes is disposed to be exposed to the second processing space. A voltage is applied between the electrodes in order to pump into the second processing space a constant amount of oxygen used for measurement of the combustible gas component contained in the measurement gas.
7. Combustible gas component concentration detection element: Formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. One of the electrodes is disposed to be exposed to the second processing space. A constant voltage is applied between the electrodes in a direction for pumping out oxygen from the second processing space, so that a current output from the combustible gas component concentration detection element varies according to the amount of oxygen consumed by combustion of the combustible gas component contained in the gas introduced from the first processing space into the second processing space to thereby provide information regarding a detected concentration of the combustible gas component of the measurement gas.
(B) Oxygen pump operation control means: Adapted to control the oxygen concentration adjustment pump element so as to reduce to a predetermined level the oxygen concentration of the measurement gas introduced into the first processing space, which is detected by the oxygen concentration detection element.
(C) Voltage application means: Adapted to apply a voltage to the combustible gas component concentration detection element.
(D) Pumping current supply means: Adapted to cause pumping current to flow between the electrodes of the measurement-purpose-oxygen supply pump element in order to pump in oxygen for measurement.
The gas sensor and the gas sensor system of the present invention sense a combustible gas component selected singly or in combination from the group consisting of, for example, hydrocarbon (HC), carbon monoxide, and hydrogen.
In the configuration described above, the oxygen concentration of the measurement gas contained in the first processing space is adjusted to a predetermined level through operation of the oxygen concentration adjustment pump element. The thus-treated gas is introduced into the second processing space, where a combustible gas component undergoes combustion through the assistance of the oxidation catalyst section. The combustible gas component concentration detection element, to which a constant voltage is applied, varies its output current according to oxygen consumption associated with the above-mentioned combustion of the combustible gas component. Thus, on the basis of the output current, information regarding the combustible gas component concentration of the original measurement gas can be determined. Since the oxygen concentration of the measurement gas detected by the oxygen concentration detection element is adjusted to a predetermined level before the gas is measured for a combustible gas component concentration, an output from the combustible gas component concentration detection element is less susceptible to the original oxygen concentration of the measurement gas. Thus, the relationship between an output current of the combustible gas component concentration detection element and the combustible gas component concentration of the measurement gas exhibits good linearity.
In the sensor of the present invention, oxygen required for burning the combustible gas component in the second processing space is supplemented by the measurement-purpose-oxygen supply pump element. Although the measurement-purpose-oxygen supply pump element seems to have a role similar to that of the second oxygen pump element of the apparatus disclosed in the above-described patent publication, the measurement-purpose-oxygen supply pump element greatly differs from the second oxygen pump element as described below. That is, in the apparatus disclosed in the patent publication, the second oxygen pump element is subjected to feedback control performed on the basis of information which is output from the oxygen concentration cell element and represents the detected oxygen concentration, and the pump current of the second oxygen pump element is detected as a combustible gas component concentration. By contrast, in the sensor of the present invention, the measurement-purpose-oxygen supply pump element supplies oxygen to the second processing space in only an amount necessary and sufficient for burning the combustible gas component, and the concentration of the combustible gas component is detected on the basis of information representing the current flowing through the combustible gas component concentration detection element. That is, unlike the apparatus disclosed in the patent publication, the sensor of the present invention does not employ a feedback system. Therefore, a variation in the concentration of the combustible gas component is directly reflected in a variation in the current of the combustible gas component concentration detection element, so that a high detection response is obtained. Thus, the above-described first object is achieved.
The rate of supply of oxygen to the second processing space is preferably maintained constant in view of accurate detection of the combustible gas component concentration. That is, the current flowing through the measurement-purpose-oxygen supply pump element is preferably maintained constant. When the combustible gas component concentration increases and a portion of the combustible gas component remains unburned, the measurement accuracy decreases. Therefore, in order to sufficiently promote combustion of the combustible gas component, the rate of supply of oxygen to the second processing space (i.e., the current flowing through the measurement-purpose-oxygen supply pump element) must be set to a proper value in accordance with the highest combustible gas component concentration to be detected. In this case, in order to guarantee supply of oxygen necessary for combustion of the combustible gas component in the second processing space, oxygen is preferably supplied to the second processing space in a slightly excess amount; i.e., the current flowing through the measurement-purpose-oxygen supply pump element is set to a slightly higher level, such that the current always flows through the combustible gas component concentration detection element in a direction for pumping out oxygen. The constant current supplied to the measurement-purpose-oxygen supply pump element preferably falls within a range of 20 to 50 xcexcA in an exemplary case where a concentration of methane is to be measured up to 1000 ppm.
Further, the constant voltage applied to the combustible gas component concentration detection element is adjusted such that the partial pressure falls within a range in which there occurs substantially no decomposition of nitrogen oxides in the gas introduced in the second processing space. This adjustment prevents a decrease in the accuracy in detecting the combustible gas component concentration that results from oxygen generated from decomposition of nitrogen oxides.
In the gas sensor of the present invention, the oxygen concentration adjustment pump element may be operated so as to adjust the oxygen concentration of the measurement gas introduced in the first processing space, which concentration is detected by the oxygen concentration detection element, within a range in which there occurs substantially no decomposition of water vapor contained in the measurement gas. In this case, the oxygen pump operation control means of the gas sensor system controls the operation of the oxygen concentration adjustment pump element such that the oxygen concentration of the measurement gas introduced in the first processing space, which concentration is detected by the oxygen concentration detection element, is adjusted within a range in which there occurs substantially no decomposition of water vapor contained in the measurement gas.
As described above, by substantially suppressing hydrogen generation due to decomposition of water vapor through oxygen concentration adjustment, there can be prevented an impairment in accuracy in measuring a combustible gas component concentration which would otherwise result from combustion of the generated hydrogen. Also, the gas sensor and the gas sensor system of the present invention exhibit excellent selectivity toward HC, particularly methane, and thus can measure a methane concentration more accurately than do conventional gas sensors. Thus, the second object of the present invention is accomplished.
When a measurement gas contains carbon dioxide and the carbon dioxide is decomposed, carbon monoxidexe2x80x94which is a combustible gas componentxe2x80x94is generated as in the case of water vapor from which hydrogen is generated. Combustion of the thus-generated carbon monoxide may lower the accuracy in detecting the combustible gas component. In such a case, it is further preferred that the oxygen concentration adjustment pump element adjust the oxygen concentration of the measurement gas introduced into the first processing space, which is detected by the oxygen concentration detection sensor, such that the oxygen concentration falls within such a range that a reaction of decomposing carbon dioxide is not substantially initiated. Since the oxygen concentration at which decomposition of carbon dioxide occurs is generally lower than the oxygen concentration at which decomposition of water vapor occurs, the decomposition of carbon dioxide is concurrently prevented through employment of the condition of oxygen concentration that prevents decomposition of water vapor.
The oxygen concentration adjustment pump element may be operated such that the oxygen concentration of the measurement gas introduced into the first processing space is adjusted within the range of 10xe2x88x9212 atm to 10xe2x88x926 atm. In this case, the oxygen pump operation control means of the gas sensor system controls the operation of the oxygen concentration adjustment pump element such that the oxygen concentration of the measurement gas introduced into the first processing space, which concentration is detected by the oxygen concentration detection element, is adjusted within the range of 10xe2x88x9212 atm to 10xe2x88x926 atm.
In the above-mentioned configuration, the oxygen concentration of the first processing space achieved through operation of the oxygen concentration adjustment pump element is adjusted within the above range, thereby suppressing decomposition of water vapor and thus improving the sensing accuracy of the gas sensor or the gas sensor system. Since an oxygen concentration to be achieved through adjustment is far higher than a conventionally required oxygen concentration of 10xe2x88x9220 atm to 10xe2x88x9214 atm, the oxygen concentration adjustment pump element carries a smaller burden even in measurement, for example, under lean-burn conditions. Thus, the service life of the oxygen concentration adjustment pump element is expanded. Also, a required power to operate the oxygen concentration adjustment pump element is not very high, and a control circuit and other peripheral devices can be formed at low cost. Also, in this case, the oxygen concentration adjustment pump element (or the oxygen pump control means) is preferably configured such that the oxygen concentration of a measurement gas introduced into the first processing space, which is detected by the oxygen concentration detection element, is adjusted such that the oxygen concentration falls within such a range that a reaction of decomposing water vapor contained in the measurement gas is not substantially initiated. In the gas sensor and the gas sensor system described above, in order to further effectively prevent decomposition of water vapor, the oxygen concentration adjustment pump element is preferably operated such that the oxygen concentration of the first processing space is adjusted to a value at which a portion of a combustible gas component is burned in the first processing space while a first electrode serves as oxidation catalyst.
Further, when a detection selectivity toward hydrocarbon (especially, methane or the like having a relatively low combustion activity) is required to be improved, the oxygen concentration within the first processing space detected by the oxygen concentration detection element is preferably adjusted such that the oxygen concentration falls within such a range that a component (e.g., carbon monoxide, hydrogen, ammonia) having a higher combustion activity than does hydrocarbon to be detected is burned more readily than is hydrocarbon to be detected. This adjustment improves the detection selectivity toward hydrocarbon (e.g. methane). The oxygen concentration range varies depending on the combustion catalytic activity of the first and fourth electrodes, which will be described later, toward various combustible gas components. However, the oxygen concentration range is 10xe2x88x9212-10xe2x88x926 atm, preferably 10xe2x88x9211-10xe2x88x929 atm.
When the oxygen concentration of a measurement gas introduced into the first processing space becomes less than 10xe2x88x9212 atm, decomposition of water vapor, if contained, becomes conspicuous. As a result, hydrogen generated through decomposition of water vapor may significantly impair accuracy in measuring a combustible gas component concentration. By contrast, when the oxygen concentration of the first processing space is in excess of 10xe2x88x926 atm, combustion of a combustible gas component becomes conspicuous in the first processing space. Accordingly, the combustible gas component concentration of a gas introduced into the second processing space becomes small with a potential failure to attain a predetermined measurement accuracy. More preferably, the oxygen concentration of the first processing space is adjusted to a value of 10xe2x88x9211 atm to 10xe2x88x929 atm.
For example, when the gas sensor is set at a working temperature of 650xc2x0 C. to 700xc2x0 C. and the water vapor concentration of a measurement gas varies within a range of about 5% to 15%, oxygen that maintains equilibrium with water vapor and hydrogen has a minimum partial pressure of about 10xe2x88x9212 atm. When the partial pressure of oxygen drops below the minimum value, decomposition of water vapor progresses, affecting accuracy in measuring a combustible gas component concentration. Therefore, in this case, the oxygen concentration of the first processing space is preferably set to a value greater than the above minimum partial pressure of oxygen.
In this specification, unless specifically described otherwise, the oxygen concentration within the first processing space means the oxygen concentration detected by the oxygen concentration detection element. For example, when a part of a combustible gas component contained in a measurement gas burns and consumes oxygen, the oxygen concentration detected by the oxygen concentration detection element is not necessarily equal to the oxygen concentration before the consumption of oxygen due to combustion. Also, the oxygen concentration may vary at locations within the first processing space due to existence of a porous electrode that is disposed to face the first processing space and catalyzes combustion of a combustible gas component, or due to oxygen pumping of the oxygen concentration adjustment pump element. In this case as well, the oxygen concentration detected by the oxygen concentration detection element is considered to represent the oxygen concentration within the first processing space.
In the gas sensor (and the gas sensor system) described above, at least either the first gas passage for introducing a measurement gas into the first processing space from outside or the second gas passage for establishing communication between the first processing space and the second processing space may be configured so as to serve as a diffusion-controlling passage for permitting gas flow at a constant diffusion resistance. This feature suppresses compositional variation of a gas introduced into the first or second processing space to a small degree of variations for a constant period of time determined by the diffusion resistance of the passage even under varying measurement gas concentration of an atmosphere subjected to measurement. Thus, accuracy in measuring a combustible gas composition concentration can be improved. Specifically, the diffusion-controlling passage may assume the form of small holes or slits or may be formed of any of various throttling mechanisms or porous metals or ceramics having communicating pores formed therein.
In the gas sensor and the gas sensor system described above, the oxygen concentration detection element may be an oxygen concentration cell element. The oxygen concentration cell element is formed of an oxygen-ion-conductive solid electrolyte and having electrodes formed on both surfaces thereof. One of the electrodes is disposed in such a manner as to be exposed to the first processing space. In this case, each of the electrode (second electrode) of the combustible gas component concentration detection element exposed to the second processing space, the electrode (third electrode) of the measurement-purpose-oxygen supply pump element exposed to the second processing space, the electrode (first electrode) of the oxygen concentration cell element exposed to the first processing space, and the electrode (fourth electrode) of the oxygen concentration adjustment pump element exposed to the first processing space may assume a form of a porous electrode having oxygen molecule dissociating capability, and at least one of the second and third electrodes may have catalytic activity for oxidation of the combustible gas component contained in the measurement gas and serves as the oxidation catalyst section. In this case, the oxidation catalytic activities of the above-described electrodes (first through fourth electrodes) are adjusted such that an amount of oxygen consumed in the second processing space due to combustion of the combustible gas component becomes greater than that in the first processing space. Thus, at least a portion of a residual combustible gas component which has not been burned in the first processing space can be reliably burned in the second processing space, thereby improving sensor sensitivity. Further, since the electrode (second electrode) of the combustible gas component concentration detection element exposed to the second processing space also serves as an oxidation catalyst section, the structure of the gas sensor or the gas sensor system is further simplified.
In view of stabilization of a sensor output, it is preferred that the electrode (second electrode) of the combustible gas component concentration detection element exposed to the second processing space be positioned in such a manner as not to interfere with the second gas passage. When the electrode interferes with the second gas passage, combustion of a combustible gas component may be initiated before equilibrium is established between a gas which is newly introduced into the second processing space from the first processing space and a gas which is already present in the second processing space. When the positional interference is avoided, such a phenomenon is less likely to occur, thereby stabilizing a sensor output.
When a gas component to be measured is CO or HC, an electrode having higher catalytic activity for oxidation may be formed of Pt, Pd, Rh, a Pt alloy, a Pd alloy, an Rh alloy, a Ptxe2x80x94Rh alloy, an Rhxe2x80x94Pd alloy, a Pdxe2x80x94Ag alloy, or a like metal (hereinafter these metals are referred to as metals of a high-activity metal group). An electrode having lower catalytic activity for oxidation may be formed of Au, Ni, Ag, an Au alloy, an Ni alloy, an Ag alloy, a Ptxe2x80x94Pd alloy, a Ptxe2x80x94Au alloy, a Ptxe2x80x94Ni alloy, a Ptxe2x80x94Ag alloy, an Agxe2x80x94Pd alloy, an Auxe2x80x94Pd alloy, or a like metal (hereinafter these metals are referred to as metals of a low-activity metal group). When a ZrO2 solid electrolyte, which will be described later, is used as an oxygen-ion-conductive solid electrolyte that constitutes a main portion of the oxygen concentration adjustment pump element, that of the oxygen concentration cell element, or that of the combustible gas component concentration detection element, a metal of the low-activity metal group to be selected is preferably the one that can be fired with the ZrO2 solid electrolyte (firing temperature: 1450xc2x0 C. to 1500xc2x0 C.), in view of improvement in sensor-manufacturing efficiency. For example, when a Ptxe2x80x94Au alloy is used, the Au contents thereof may be 0.1% to 3% by weight. When the Au content is less than 0.1% by weight, an electrode formed of the alloy may have an excessively high catalytic activity for oxidation.
In the gas sensor of the present invention, a more preferable result is obtained through employment of an electrode having the following structure. Specifically, the oxygen concentration adjustment pump element is formed of an oxygen-ion-conductive solid electrolyte and has electrodes formed on both surfaces thereof, and one of the electrodes (hereinafter referred to as the xe2x80x9cfourth electrodexe2x80x9d) is disposed in such a manner as to be exposed to the first processing space. When a component to be detected is CO or HC, the fourth electrode is composed of two layers, i.e., a porous main electrode layer and a porous surface electrode layer. The porous main electrode layer is made of Ptxe2x80x94Au alloy or Pt. The porous surface electrode layer covers the main electrode layer to thereby form a surface layer portion of the fourth electrode. The surface electrode layer is made of a material selected from the group consisting of a metal containing Au or Ag as a main component, Ptxe2x80x94Au alloy, Auxe2x80x94Pd alloy, Ptxe2x80x94Ag alloy, and Ptxe2x80x94Ni alloy (hereinafter collectively referred to as xe2x80x9cinactive metalxe2x80x9d). In this specification, the term xe2x80x9cX-Y alloyxe2x80x9d means an alloy in which a metal component having a highest content by weight is X, and a metal component having a second highest content by weight is Y, and may be an X-Y binary system alloy or a higher-order system alloy containing X, Y, and other alloy components.
Materials for the electrodes of the oxygen concentration cell element or the oxygen concentration adjustment pump element must have a sufficient catalytic activity for dissociation and recombination of oxygen molecules. Pt single metal, for example, is an excellent material in this point. However, if this material is used for the electrode exposed to the first processing space, the material has an extremely high combustion catalytic activity toward a combustion gas component. Therefore, the catalytic activity must be decreased slightly. For example, as conventionally practiced, Au, whose combustion catalytic activity is low, is mixed to Pt up to about 20 wt. %, thereby forming a Ptxe2x80x94Au alloy. However, when the Au content increases, a drastic decrease in the activity for dissociating oxygen molecules occurs concurrently with a decrease in the catalytic activity for combustion of a combustible gas component. Therefore, these two catalytic activities are difficult to balance.
This problem can be solved through employment of the above-described multilayer electrode, in which the surface of the porous main electrode layer formed of Ptxe2x80x94Au alloy or Pt having a high activity for dissociating oxygen molecules is covered with the porous surface electrode layer formed of an inactive metal having a low combustion catalytic activity toward a combustible gas component. This structure enables convenient adjustment to decrease the combustion catalytic activity toward a combustible gas component to a possible extent, while maintaining a sufficient level of oxygen molecule dissociation activity.
In the present invention, the surface electrode layer is preferably formed of an Au-containing porous metal that has a considerably low combustion catalytic activity toward CO or HC and some degree of catalytic activity for dissociation and recombination of oxygen molecules. However, there may alternatively be used a porous metal containing Ag as a main component, a porous Ptxe2x80x94Au alloy (Au content: 5 wt. % or more), a porous Ptxe2x80x94Pb alloy (Pb content: 1 wt. % or more), a porous Ptxe2x80x94Ag alloy (Ag content: 1 wt. % or more), a porous Ptxe2x80x94Ni alloy (Ni content: 1 wt. % or more), and the like.
The surface electrode layer and the main electrode layer may be arranged such that these layers come into indirect contact with each other via one or more other layers. However, employment of a two-layer structure comprising the main electrode layer and the surface electrode layer simplifies the manufacturing process. In this case, when the surface electrode layer is formed of an Au-containing porous metal that contains Au as a main component, there can be obtained the remarkable effect of suppressing the combustion catalytic activity toward a combustible gas component, while there is maintained a sufficient level of oxygen molecule dissociation activity.
The above-described multilayer electrode is advantageously employed for the fourth electrode of the oxygen concentration adjustment pump element which is not required to have a sharp response to oxygen concentration. It is not impossible to use the above-described multilayer electrode for the first electrode of the oxygen concentration cell element. However, in order to further improve the accuracy in detecting the oxygen concentration within the first processing space by use of the oxygen concentration cell element, the first electrode is preferably formed of Pt, Ptxe2x80x94Au alloy, or Ptxe2x80x94Ag alloy. In this case, since combustion of a combustible gas component that is caused by the first electrode within the first processing space can be suppressed by making the area of the first electrode smaller than that of the fourth electrode, the loss caused by the combustion of the combustible gas component within the first processing space can be decreased, so that the sensor sensitivity can be increased further.
When Ptxe2x80x94Au alloy or Ptxe2x80x94Ag alloy is used for the first electrode, Au or Ag is added in order to suppress the combustion catalytic activity toward CO or HC. In this case, when the Au or Ag content exceeds 1 wt. %, the oxygen molecule dissociation activity decreases excessively, resulting in a deterioration of the oxygen concentration detection performance. By contrast, when the Au or Ag content is less than 0.1 Wt. %, almost no effect of suppressing the combustion catalytic activity is expected. Au and Ag may be added together into Pt such that their total content does not exceed 1 wt. %.
When a detection selectivity for hydrocarbon among various combustible gas components must be improved, a component having a higher combustion activity than does hydrocarbon to be detected is preferably burned more readily than is hydrocarbon to be detected. In this case, as described above, the oxygen concentration within the first processing space detected by the oxygen concentration detection element is adjusted. Further, the combustion catalytic activity of the first electrode or the fourth electrode exposed to the first processing space and the temperature within the first processing space are important factors in improving the detection selectivity. When the fourth electrode is formed of the above-described multilayer electrode having a relatively low combustion catalytic activity and the first electrode is formed of Pt or Pt alloy having a high combustion catalytic activity, a hydrocarbon component (e.g., methane) having a slightly low combustion activity does not burn much, while components such as carbon monoxide, hydrogen, ammonia, which have a higher combustion activity, burn readily on the first electrode. As a result, there is created an environment convenient for selective detection of hydrocarbon components. When the temperature within the first processing space increases, combustion reaction proceeds easily, and a difference in combustion catalytic activity is not produced so much between electrodes of different materials. These are disadvantageous for selective detection of hydrocarbon components. However, when the fourth electrode has the above-described multilayer structure, a considerably large difference in catalytic activity between the fourth electrode and the first electrode formed of Pt or the like is produced even at considerably high temperatures (e.g. 700-800xc2x0 C.), so that selective detection of hydrocarbon components can be performed effectively.
When the fourth electrode is formed into the above-described multilayer structure, the gas sensor of the present invention can be manufactured in accordance with the method comprising the following steps.
(1) A substrate electrode layer forming step in which a substrate electrode pattern containing an unfired layer of material powder for the main electrode layer of the fourth electrode (hereinafter referred to as the xe2x80x9cunfired main electrode layerxe2x80x9d) is formed on an unfired compact of the oxygen-ion-conductive solid electrolyte layer contained in the oxygen concentration adjustment pump element (hereinafter referred to as the xe2x80x9cunfired solid electrolyte compactxe2x80x9d), and the unfired main electrode layer is integrally fired with the unfired solid electrolyte compact in order to form on the oxygen-ion-conductive solid electrolyte layer a substrate electrode layer containing the main electrode layer.
(2) A surface electrode layer forming step in which a layer of material powder for the surface electrode layer is formed on the substrate electrode layer, and is subjected to a secondary firing at a temperature lower than that in the integral firing to thereby form the surface electrode layer. The layer of material power may be formed through, for example, application of paste of the material powder onto the main electrode layer.
Since the substrate electrode layer containing the main electrode layer is formed of a high-melting point metal such as Pt or a Ptxe2x80x94Au or Ptxe2x80x94Ag alloy having the above-described composition, the substrate electrode layer can be fired concurrently with solid electrolyte ceramic, such as zirconia, that constitutes the main portion of each element. However, when the surface electrode layer is formed of an Au-containing metal, which has a low melting point, maintaining the porous state of the substrate electrode layer becomes impossible when it is fired together with solid electrolyte ceramic. In addition, Au diffuses into the substrate electrode layer, and therefore it becomes impossible to achieve the effect of suppressing the combustion catalytic activity. In order to solve this problem, there is employed the above-described process in which the surface electrode layer is subjected to secondary firing at a temperature lower than that for the integral firing of the substrate electrode layer and the solid electrolyte layer in order to bond the surface electrode layer to the substrate electrode layer through baking. Thus, a multilayer electrode having a desired performance is obtained.
The components (e.g., Au) of the surface electrode layer may diffuse into the main electrode layer during the secondary firing or when the sensor is used at high temperature. For example, even if the main electrode layer is substantially formed of Pt, Au may diffuse from the surface electrode layer into the main electrode layer so that Au constituting the main electrode layer is converted into Ptxe2x80x94Au alloy. If the diffusion of the material of the surface electrode layer into the main electrode layer proceeds excessively, the thickness of the surface electrode layer becomes insufficient, or in an extreme case, the surface electrode layer disappears. For example, when it is desired that the surface electrode layer be formed mainly of Au and the main electrode layer be formed mainly of Pt, the temperature for secondary firing is preferably set to about 800-1050xc2x0 C. in order to prevent excessive diffusion of Au into the main electrode layer. When the secondary firing temperature is less than 800xc2x0 C., firing of the surface electrode layer becomes insufficient with a possible result that delamination of the surface electrode layer occurs due to insufficient closeness of contact. By contrast, when the secondary firing temperature is greater than 1050xc2x0 C., the thickness of the surface electrode layer becomes insufficient due to diffusion of the Au component, or firing proceeds excessively, so that the porous structure is lost. In this case, the oxygen permeability that the porous electrode must have becomes difficult to maintain. When Au is mixed in the constituent metal of the main electrode layer in an amount of about 3xe2x88x9210 wt. % from the beginning, the diffusion of Au from the surface electrode layer into the main electrode layer can be suppressed because the limit of solid solution of Au into Pt is relatively small (about 5 wt. % at 800xc2x0 C.). Thus, the drawbacks such as a reduction in thickness of the surface electrode layer can be effectively avoided.
The manufacturing method comprising the above-described secondary firing step can be performed efficiently, when the gas sensor of the present invention is constructed such that a pump cell unit including the oxygen concentration adjustment pump element is formed separately from a sensor cell unit including the oxygen concentration detection element, the second processing space, and the combustible gas component concentration detection element; and the pump cell unit and the sensor cell unit are joined and integrated with each other through use of a bonding material. In this case, the pump cell unit is manufactured through firing such that the substrate electrode layer is formed without formation of the surface electrode layer; the secondary firing is performed in order to form the surface electrode layer on the substrate electrode layer of the pump cell unit; and the pump cell unit is integrated with the sensor cell unit, which has been separately manufactured through firing. Thus, the gas sensor is obtained. Preferably, a pump-cell-side fitting portion is formed in the pump cell unit, and a sensor-cell-side fitting portion to be engaged with the pump-cell-side fitting portion is formed in the sensor cell unit. In this case, positioning during joining can be easily performed through engagement between the pump-cell-side fitting portion and the sensor-cell-side fitting portion. Thus, manufacturing efficiency of the sensor can be improved.
In the gas sensor of the present invention, the oxygen concentration cell element or the combustible gas component concentration detection element may be formed of an oxygen-ion-conductive solid electrolyte composed mainly of ZrO2 (ZrO2 solid electrolyte). In the oxygen concentration cell element formed of a ZrO2 solid electrolyte, one electrode is in contact with a gas to be measured, which gas contains oxygen and a combustible gas component, while the other electrode is in contact with a reference atmosphere having a constant oxygen concentration. An electromotive force of the oxygen concentration cell element varies abruptly when a gas composition falls outside a stoichiometric composition in which oxygen and a combustible gas component are present in a proper ratio so that they react with each other completely. When an ordinary gasoline engine or diesel engine is operated under lean-burn conditions, a measurement gas emitted from the engine contains combustible gas components in a total concentration of about 0 to 1000 ppmC (ppmC: parts per million represented with carbon equivalent). A measurement gas having such a combustible gas component concentration is introduced into the first processing space, and the oxygen concentration of the introduced measurement gas is adjusted to 10xe2x88x926 atm (preferably 10xe2x88x929 atm) or lower, as mentioned previously. As a result, a gas introduced into the second processing space from the first processing space has a stoichiometric composition or a composition shifted slightly toward a rich condition. Thus, an output from the combustible gas component concentration detection element is increased, thereby improving the sensitivity of the gas sensor.
When the oxygen concentration adjustment pump element, the oxygen concentration cell element, and the combustible gas component concentration detection element are formed of the ZrO2 solid electrolyte mentioned above, a heating element may be provided for heating the elements to a predetermined working temperature. The working temperature may be set to 650xc2x0 C. to 800xc2x0 C. When the working temperature is in excess of 800xc2x0 C., an output current value of the combustible gas component concentration detection element becomes excessively low, causing impairment in the sensitivity of the gas sensor. This is conceivably because most of a combustible gas component, such as an HC component, contained in a measurement gas is burned in the first processing space due to the working temperature being high. By contrast, when the working temperature is lower than 650xc2x0 C., the internal resistance of the oxygen concentration adjustment pump element increases, causing unstable operation. As a result, accuracy in measuring a combustible gas component may be impaired.
In the gas sensor and the gas sensor system of the present invention, the oxygen concentration detection element may be an oxygen concentration cell element formed of an oxygen-ion-conductive solid electrolyte and having porous metal electrodes formed on both surfaces thereof. One of the electrodes (hereinafter referred to as the xe2x80x9cfirst electrodexe2x80x9d) may serve as a detection-side electrode disposed in such a manner as to be exposed to said first processing space, and the other electrode may serve as an oxygen reference electrode. In this case, the oxygen reference electrode of the oxygen concentration detection element is used as the electrode of the combustible gas component concentration detection element opposite the electrode (hereinafter referred to as the xe2x80x9csecond electrodexe2x80x9d) thereof exposed to the second processing space. This arrangement enables the oxygen concentration detection element and the combustible gas component concentration detection element to share the oxygen reference electrode, thereby implementing a compact sensor.
More specifically, the first processing space and the second processing space may be arranged with a partition wall, formed of an oxygen-ion-conductive solid electrolyte, disposed therebetween. In this case, the second gas passage is formed in the partition wall so as to establish communication between the first processing space and the second processing space. An oxygen reference electrode is embedded in the partition wall at a thicknesswise intermediate portion. The first electrode is formed on the partition wall in such a manner as to be exposed to the first processing space. The oxygen concentration cell element is constituted by the first electrode, the oxygen reference electrode, and a portion of the partition wall interposed between the first electrode and the oxygen reference electrode. Also, the second electrode is formed on the partition wall in such a manner as to be exposed to the second processing space. The combustible gas component concentration detection element is constituted by the second electrode, the oxygen reference electrode, and a portion of the partition wall interposed between the second electrode and the oxygen reference electrode. The oxygen concentration adjustment pump element is arranged opposite to the partition wall with the first processing space disposed therebetween. This arrangement enables all the elements to be formed in a laminated configuration, to thereby make the sensor more compact.
The oxygen reference electrode of the oxygen concentration cell element may be a self-generation-type oxygen reference electrode. That is, a small pumping current is caused to flow between the detection-side electrode and the oxygen reference electrode of the oxygen concentration cell element in a direction such that oxygen is pumped toward the oxygen reference electrode, so that the thus-pumped oxygen establishes a predetermined reference oxygen concentration within the oxygen reference electrode. Thus, the oxygen concentration at the oxygen reference electrode side can be stabilized, so that oxygen concentration can be measured with higher accuracy.
When the oxygen concentration detection element and the combustible gas component concentration detection element shares the oxygen reference electrode, and the oxygen reference electrode is configured to serve as the self-generation-type oxygen reference electrode, a current limit circuit is preferably provided in order to prevent an amount of current flowing through the combustible gas component concentration detection element between the second electrode and the oxygen reference electrode from falling outside a predetermined range. If such an excessive current flows from the oxygen reference electrode to the second electrode, a large amount of oxygen flows toward the oxygen reference electrode, so that the internal pressure of the oxygen reference electrode becomes excessively high, resulting in occurrence of problems such as breakage of the electrode. Such problems can be avoided through provision of the above-described current limit circuit.
The current limit circuit may be configured to prevent an amount of current flowing from the oxygen reference electrode to the second electrode from exceeding a predetermined value. The fact that current flows from the second electrode to the oxygen reference electrode means that oxygen flows out from the oxygen reference electrode toward the second electrode, because the oxygen-ion-conductive solid electrolyte is present therebetween. When such a current flows excessively, a large amount of oxygen flows out from the oxygen reference electrode, so that the oxygen reference electrode becomes difficult to secure a necessary oxygen concentration. As a result, it becomes impossible to operate the oxygen concentration cell element properly or normally, or to control the oxygen concentration in the first processing space, resulting in a decreased detection accuracy of the sensor. This drawback can be avoided through provision of the above-described current limit circuit.
Preferably, at least either the oxygen reference electrode or the second electrode is formed in or on the partition wall at such a position as not to interfere with the second gas passage. More preferably, both of them are positioned in such a manner as not to interfere with the second gas passage. Positioning the second electrode in such a manner yields the aforementioned advantage. Also, positioning the oxygen reference electrode in such a manner prevents leakage of oxygen from the oxygen reference electrode through the second gas passage, thereby stabilizing an oxygen reference concentration and thus stabilizing a sensor output indicative of a combustible gas component concentration.
In the above gas sensor, the current flowing through the oxygen pump element, i.e., an oxygen pump current, varies according to the oxygen concentration of a measurement gas. Accordingly, the oxygen pump current reflects the oxygen concentration of the measurement gas. Therefore, in the gas sensor system of the present invention, there may be provided correction means for correcting an output of the combustible gas component concentration detection element based on the oxygen concentration of a measurement gas reflected in the oxygen pump current. That is, as mentioned previously, the gas sensor system of the present invention is characterized by having less susceptibility to the oxygen concentration of a measurement gas. Nevertheless, when the oxygen concentration causes variations in the output, such variations can be corrected by the correction means, thereby further improving accuracy in measuring a combustible gas concentration.
Specifically, the correction means may include storage means and correction value determination means. The storage means stores information regarding the relation between an output current of the combustible gas component concentration detection element and a combustible gas component concentration, relative to various values of oxygen concentration (or values of oxygen pump current). The correction value determination means determines a corrected output current (or a corresponding combustible gas component concentration) based on an output current of the combustible gas component concentration detection element and the above information. Thus, a detected combustible gas component concentration can be corrected in a rational manner.